A standard method to measure the arterial oxygenation of blood is known as pulse oximetry. Pulse oximeters function on the basis that at differing wavelengths, blood attenuates light very differently depending upon the level of oxygenation. Pulse waves starting from the heart cause in the arterial blood vessel system a periodic fluctuation in the arterial blood content in the tissue. As a consequence, a periodic change in the light absorption (FIG. 1) can be registered between the light transmitter, whose radiation passes through the tissue, and the receivers, which are integrated in a pulse oximetry sensor. The evaluation of the sensor signals is normally carried out at light wavelengths of w1=660 and w2=940 nm by calculating the differential change of light absorption. It is possible to create a measured variable R which is obtained in the following manner or in a similar manner:
                              R          =                      Rw            ⁢                                                  ⁢            1                          ,                              w            ⁢                                                  ⁢            2                    =                                                    Δ                ⁡                                  (                                      LAw                    ⁢                                                                                  ⁢                    1                                    )                                            =                                                ln                  ⁡                                      (                                                                  I                        ⁢                                                                                                  ⁢                        max                                            ,                                              w                        ⁢                                                                                                  ⁢                        1                                                              )                                                  -                                  ln                  ⁡                                      (                                                                  I                        ⁢                                                                                                  ⁢                        min                                            ,                                              w                        ⁢                                                                                                  ⁢                        1                                                              )                                                                                                                        Δ                  ⁡                                      (                                          LAw                      ⁢                                                                                          ⁢                      2                                        )                                                  ⁢                                  ln                  ⁡                                      (                                                                  I                        ⁢                                                                                                  ⁢                        max                                            ,                                              w                        ⁢                                                                                                  ⁢                        2                                                              )                                                              -                              ln                ⁡                                  (                                                            I                      ⁢                                                                                          ⁢                      min                                        ,                                          w                      ⁢                                                                                          ⁢                      2                                                        )                                                                                        Eq        ⁢                  :                ⁢                                  ⁢                  (          1          )                    
The light intensities described in the formula represent the light intensities received in the receiver of the sensors used in pulse oximetry. The measured variable R serves as a measurement for the oxygen saturation. The formation of a quotient in order to form the measured variable is intended to compensate for any possible influences the haemoglobin content of the tissue, the pigmentation of the skin or the pilosity may have on the measurement of the oxygen saturation of arterial blood. The difference of the light attenuations at a minimum and maximum value is the delta of the light attenuations for each of both wavelengths.
Measuring oxygen saturation of arterial blood in the tissue in a range of 70 to 100% using light of wavelength 940 nm and 660 nm most often produces for one single application site sufficiently accurate measured values. However, in order to measure lower oxygen saturation of arterial blood it is necessary to assume a strong influence on the measured variable R in particular caused by perfusion (i.e. blood content) (see: IEEE; Photon Diffusion Analysis of the Effects of Multiple Scattering on Pulse Oximetry by J. M. Schmitt; 1991) and other optical parameters of tissue.
U.S. Pat. No. 5,529,064 to Rall, describes a fetal pulse oximetry sensor. For this kind of application, a higher measurement precision is desirable because a fetus has a physiological lower oxygenation than adult human beings and measurement error of SaO2 increases at low oxygenations.
U.S. Pat. No. 6,226,540 to Bernreuter, incorporated by reference herein, improves the precision of pulse oximetry. However, in order to measure on different body sites with the same high resolution for the arterial oxygenation, additional precision to measure optical tissue properties is necessary. Another problem is that pulse oximetry alone does not provide sufficient diagnostic information to monitor critically ill patients (See: When Pulse Oximetry Monitoring of the Critically Ill is Not Enough by Brian F. Keogh in Anesth Analg (2002), 94:96-99).
Because of this it would be highly desirable to be able to additionally measure the mixed venous oxygenation of blood SvO2. Methods to measure SvO2 with NIR were described by Jöbsis in U.S. Pat. No. 4,223,680 and by Hirano et al in U.S. Pat. No. 5,057,695. A problem of those disclosed solutions is that hair, dirt or other optically non-transparent material on the surface of tissue can influence the measured results for SvO2.
To measure the metabolism of blood oxygenation, Anderson et al in U.S. Pat. No. 5,879,294 disclose an instrument in which the second derivative of the light spectrum used delivers information about the oxygenation. Hereby, the influence of light scattering in tissue is minimized, which can result in higher measurement precision. A disadvantage of this solution is that the calibration of the optical instruments is complicated and expensive, which makes it impractical to use such devices for sports activity applications, where light weight wearable devices would be of interest. Similar problems are known for frequency domain spectroscopy disclosed for example in Gratton, U.S. Pat. No. 4,840,485. Oximetry devices, which are described in the present specification and which simply measure light attenuations of tissue at different wavelengths, are more feasible, flexible and reliable in practice than complex time resolved methods.